An emissive cathode is an electronic device that emits electrons during processes such as field emission, thermionic emission, and photoemission. For example, in field emission, electrons are extracted from an emissive cathode when it is subjected to an electric field of sufficient strength. The electric field is created by applying a voltage between the cathode and an electrode, commonly referred to as the anode or gate electrode, situated a short distance away from the cathode.
Emissive cathodes have been made from tungsten substrates with thin films of cesium deposited over the tungsten. The cesium lowers the cathode work function so as to increase the emission of electrons. Unfortunately, the cesium in a Cs-W cathode tends to evaporate if the cathode is subjected to high temperature, typically 200.degree. C. or more. The electron emissivity severely degrades.
Langmuir et al, "Oxygen Films on Tungsten. I. A Study of Stability by Means of Electron Emission in Presence of Cesium Vapor," J. Amer. Chem. Soc., Feb. 1931, pp. 487-497, found that the evaporation problem could be substantially overcome by installing a thin layer, essentially a monolayer, of oxygen between the tungsten and cesium. Oxygen forms very strong chemical bonds with both tungsten and cesium. As a result, exposure at high temperature would not cause the cesium to evaporate away.
In addition, the work function of a Cs-0-W cathode is usually less than that of an otherwise equivalent Cs-W cathode despite the fact that oxygen by itself increases the work function. The interaction of cesium and oxygen more than compensates for the work function-increasing effect of oxygen. Consequently, a Cs-0-W cathode also has a greater electron emissivity than a Cs-W cathode.
Lin, "The role of oxygen and fluorine in electron emission of some kinds of cathodes," J. Vac. Sci. Technol., May/June 1988, pp. 1053-1057, investigated an emissive cathode in which a thin barium film was used in place of the cesium film to form a Ba-0-W cathode having a low work function. Lin also considered replacing the tungsten substrate in his Ba-0-W cathode with another 6s.sup.2 metal or with a 6s.sup.1 metal such as gold or platinum.
Macaulay et al, "Cesiated thin-film field-emission microcathode arrays," Appl. Phys. Lett., 24 Aug. 1992, pp. 997-999, found that forming thin cesium coatings on molybdenum cathode tips reduced the work function. U.S. Pat. No. 5,089,292, also to Macaulay et al, discloses a technique for creating such Cs-coated cathodes. In this patent, Macaulay et al specify that the Cs coating could be replaced with a coating of barium, thorium, strontium, sodium, or potassium.
Emissive cathodes utilizing semiconductor substrates have been investigated. For example, Rougeot et al, "Negative Electron Affinity Photoemitters," Adv. Electronics & Electron Phys., Vol. 48, 1979, pp. 1-36, reported on emissive cathodes having gallium arsenide substrates coated with composite films of cesium and oxygen. Levine, "Structural and Electronic Model of Negative Electron Affinity on the Si/Cs/O Surface," Surf. Sci., 1973, pp. 90-107, examined a complex coating of cesium and oxygen on a silicon substrate.
Emissive cathodes are..employed in cathode-ray tubes of both the conventional conical type and the new flat-panel type. One requirement for certain flat-panel applications is that an emissive cathode be sealed in a highly evacuated tube at 400.degree. C. It is desirable that the fabrication of the cathode be complete before being placed in the tube so as to avoid contamination from materials used in fabricating the cathode. Consequently, the cathode must be capable of being subjected to one atmosphere of air for 1-2 days without undergoing significant degradation as time passes. The foregoing prior art emissive cathodes are not capable of meeting these specifications while simultaneously having long lifetimes and high emissivities that can be readily controlled during cathode fabrication.